LibGUI+WindowServer: Start fleshing out drag&drop functionality

This patch enables basic drag&drop between applications.
You initiate a drag by creating a GDragOperation object and calling
exec() on it. This creates a nested event loop in the calling program
that only returns once the drag operation has ended.

On the receiving side, you get a call to GWidget::drop_event() with
a GDropEvent containing information about the dropped data.

The only data passed right now is a piece of text that's also used
to visually indicate that a drag is happening (by showing the text in
a little box that follows the mouse cursor around.)

There are things to fix here, but we're off to a nice start. :^)

Libraries/LibGUI/GWindowServerConnection.cpp

@@ -2,6 +2,7 @@
 #include <LibGUI/GApplication.h>
 #include <LibGUI/GClipboard.h>
 #include <LibGUI/GDesktop.h>
+#include <LibGUI/GDragOperation.h>
 #include <LibGUI/GEvent.h>
 #include <LibGUI/GMenu.h>
 #include <LibGUI/GWidget.h>
@@ -261,3 +262,19 @@ void GWindowServerConnection::handle(const WindowClient::AsyncSetWallpaperFinish
 {
     // This is handled manually by GDesktop::set_wallpaper().
 }
+
+void GWindowServerConnection::handle(const WindowClient::DragDropped& message)
+{
+    if (auto* window = GWindow::from_window_id(message.window_id()))
+        CEventLoop::current().post_event(*window, make<GDropEvent>(message.mouse_position(), message.text()));
+}
+
+void GWindowServerConnection::handle(const WindowClient::DragAccepted&)
+{
+    GDragOperation::notify_accepted({});
+}
+
+void GWindowServerConnection::handle(const WindowClient::DragCancelled&)
+{
+    GDragOperation::notify_cancelled({});
+}